While drilling boreholes, various types of equipment failure may occur. Such failures may result in blocking the borehole with a heavy steel part which prevents further drilling. For example, a drill bit may break or twist off the bottom of a drill string and become stuck in the borehole. Drill bits are made of very hard materials since they are designed to drill through rock. It is difficult to impossible to reliably use another drill bit to drill through a broken bit in a well.
One method of clearing a non-drillable obstruction in a borehole has been to use a large shaped charge to break the obstruction into small pieces which can be cleared from the hole by circulating fluid and/or which can be drilled through. Such charges are commonly referred to as junk shots. Originally, a junk shot was attached to the end of a wireline and lowered to the obstruction. The wireline provided electrical connections which were used to actuate an electrically fired firing head which in turn detonated the shaped charge. The use of wirelines to deliver junk shots was often not effective for several reasons. It is often difficult to know exactly how deep a wireline has conveyed the junk shot into the borehole. The junk shot charge should be very close to the obstruction, preferably in direct contact with the obstruction. Since a wireline does not provide reliably accurate depth indications, the charge may not be properly placed. Since wireline equipment is not normally kept at a well location during drilling, it may be several days after an obstruction occurs before wireline equipment can be run down a borehole. In the meantime, solids from drilling mud, the borehole walls, etc. may settle on top of the obstruction. A column of such solids may prevent the junk shot from reaching the obstruction and can effectively shield the obstruction from the full force of the charge when it is fired. A wireline is generally only useful in generally vertical boreholes since it depends on gravity to lower the junk shot into the borehole.
In view of the problems wit using wirelines to convey junk shots into boreholes, systems were devised to convey the junk shots on the end of a drill string or other work string. Since the blockage usually occurs during drilling, a drill string is usually available. In addition, drilling mud could be circulated down the drill string to clear debris which may have settled on the obstruction. Standard practice has been to attach a junk shot to the bottom of the drill or work string without a detonator and lower the string to the desired depth while circulating mud. The use of a drill string allows accurate measurement of depth, even in horizontal boreholes. Once the charge is positioned on the obstruction, an activating charge, e.g. an electrically activated firing head, has been conveyed, typically on a wireline, down through the drill string to the main junk shot. Once it is in place, the activating charge is fired, which then fires the main junk shot. This system also has several problems. The main charge is subjected to downhole pressure and temperature for prolonged times as the activating charge is prepared and conveyed downhole. These conditions may cause failure of the main charge. The extra time and equipment needed to convey the activating charge into the borehole greatly increases the cost of the operation.